Method for improving methane production through low-temperature anaerobic digestion of sewage sludge

Abstract

The invention discloses a method for improving methane production through low-temperature anaerobic digestion of sewage sludge, and relates to the technical field of solid waste resource utilization and renewable energy sources. The invention aims to solve the problem of low methane yield in the anaerobic digestion process of sewage sludge under a low-temperature condition. The method comprises the following steps: adding an inoculum and a culture medium into a reaction device, and fully stirring and mixing to obtain a mixed matrix; the inoculum is anaerobic digestion sludge, and the culture medium is sewage sludge which is subjected to heat treatment at 90-100 DEG C for 0.5-1 hour; and adding the biochar into the mixed matrix, and carrying out low-temperature anaerobic digestion to complete the improvement of the low-temperature anaerobic digestion methane production of the sewage sludge. The method for improving methane production through low-temperature anaerobic digestion of sewage sludge can be obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of solid waste resource utilization and renewable energy, and in particular to a method for improving methane production by low-temperature anaerobic digestion of sewage sludge. Background Art

[0002] As a mature means of waste resource utilization, anaerobic digestion technology can convert complex organic matter into methane-rich biogas (biogas) under the action of microorganisms, while achieving the reduction and stabilization of pollutants. It is recognized as one of the core technologies for realizing the resource utilization of organic waste. However, the widespread promotion and application of anaerobic digestion technology still faces many challenges, mainly including low biogas yield and poor stability. The above challenges are particularly prominent under low temperature conditions (usually 20°C), which greatly limits the effective application of anaerobic digestion technology in cold climate areas or seasonal low temperature periods. Therefore, how to effectively improve the methane production efficiency of the anaerobic digestion process under low temperature conditions, especially for the common organic waste such as the large amount of sewage sludge generated by sewage treatment plants, is a key technical problem and research hotspot that needs to be solved in the current anaerobic digestion field.

[0003] In recent years, studies have shown that adding conductive materials such as biochar to anaerobic digestion systems may have a positive effect on improving biogas production. It is speculated that the porous structure, surface functional groups, and conductive properties of biochar may improve the anaerobic digestion process by enriching microorganisms, increasing the activity of key enzymes, regulating the structure of functional microbial communities, and promoting electron transfer, especially alleviating problems such as low gas production efficiency and poor system stability. Despite this, it is still unclear whether biochar can promote the methane production efficiency of anaerobic digestion of sewage sludge under low-temperature conditions. Therefore, the development of an enhanced technology for low-temperature anaerobic digestion of sewage sludge, in order to achieve efficient methane production and rational utilization of waste resources under low-temperature environments, has important theoretical significance and practical application value. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem of low methane production during anaerobic digestion of sewage sludge under low temperature conditions and to provide a method for improving methane production during low-temperature anaerobic digestion of sewage sludge.

[0005] A method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge is carried out according to the following steps:

[0006] Step S1, preparing a mixed matrix:

[0007] Adding the inoculum and culture medium into the reaction device, stirring and mixing them thoroughly to obtain a mixed matrix;

[0008] The inoculum is anaerobic digested sludge, and the culture medium is sewage sludge that has been heat-treated at 90-100°C for 0.5-1 h;

[0009] Step S2, low-temperature anaerobic digestion to produce methane:

[0010] The biochar is added to the mixed matrix obtained in step S1 to perform low-temperature anaerobic digestion, thereby improving the methane production by low-temperature anaerobic digestion of sewage sludge.

[0011] Principle of the present invention:

[0012] In the past, the sludge heat treatment temperature was usually between 70 and 200°C. Although this high temperature can significantly increase the release of soluble COD and VFA, it also produces inhibitory by-products such as bran and phenols, which put toxic pressure on the subsequent microbial system and bring higher energy consumption and equipment safety risks.

[0013] In contrast, the present invention's heat treatment, at 90-100°C, falls within the medium-low temperature range and offers a better technical balance: on the one hand, it promotes the release of soluble carbon sources; on the other hand, within this temperature range, it is less likely to produce intermediates that inhibit anaerobic bacteria. Furthermore, this temperature range reduces energy consumption and enhances process safety. Therefore, heat treatment at this temperature strikes a good balance between improving substrate availability and ensuring the stability of the microbial system, making it a preferred pretreatment solution for subsequent anaerobic digestion processes.

[0014] At low temperatures (typically below 25°C), anaerobic digestion processes often experience a significant decrease in methane yield due to reduced microbial metabolic activity and limited hydrolysis and acidification, limiting the technology's effectiveness in cold regions or during winter operations. To address this, this paper proposes the use of biochar as a functional additive. Leveraging its unique physicochemical properties, this additive synergistically enhances methane production and system stability within low-temperature anaerobic digestion systems.

[0015] Specifically, biochar has a rich pore structure and a large specific surface area, and can serve as an attachment carrier for methanogens and related functional bacteria, effectively increasing the concentration of microorganisms and enhancing system stability; biochar has good conductivity, which can promote direct extracellular electron transfer (DIET) between different microorganisms, significantly accelerating the conversion rate of intermediate metabolites to methane, and is especially helpful in improving the problem of insufficient hydrogen utilization efficiency at low temperatures; in addition, the surface of biochar contains a variety of alkaline functional groups, which have the ability to buffer pH fluctuations and adsorb inhibitory substances (such as ammonia nitrogen and heavy metal ions), thereby slowing down acidification imbalance and improving the system's impact resistance; at the same time, biochar can also activate or maintain the metabolic activity of methanogens in low temperature environments, showing good low-temperature adaptability.

[0016] In summary, the present invention achieves a significant increase in methane yield, shortened startup time, and enhanced system stability and anti-interference capabilities during low-temperature anaerobic digestion through the multiple action mechanisms of biochar, providing an efficient, economical, and environmentally friendly solution for the application of anaerobic technology in low-temperature areas or areas with seasonally low temperatures.

[0017] Beneficial effects of the present invention:

[0018] (1) The present invention successfully overcomes the inhibitory effect of low temperature on microbial metabolic activity by innovatively introducing biochar into a low-temperature anaerobic digestion system, combined with an optimized substrate pretreatment method. As shown in Example 1, under the conditions of optimal biochar dosage, the methane yield of the system increased significantly by 254.8% compared to the blank control group without biochar addition, indicating a breakthrough improvement in the energy conversion efficiency of sewage sludge under low-temperature conditions.

[0019] (2) The biochar added in the present invention not only serves as an excellent carrier for microbial attachment, but more importantly, it serves as an efficient exogenous electron mediator, which can significantly improve the activity of key metabolic enzymes in the anaerobic digestion system, optimize and regulate the community structure and abundance of core functional microorganisms such as acid-producing bacteria, methanogens and electroactive bacteria, promote the synergistic effect and direct / indirect electron transfer efficiency among microorganisms, and build a more stable and efficient microbial metabolic network, thereby greatly enhancing the operational robustness and shock resistance of the entire anaerobic digestion system under adverse conditions such as low temperature.

[0020] (3) The technical solution proposed in the present invention has clear steps and a simple and easy operation process. It is highly compatible with existing sewage sludge anaerobic digestion facilities and can be applied and promoted without large-scale or complex equipment modifications. At the same time, biochar, as the core strengthening material, is widely available and relatively inexpensive. This allows the method of the present invention to achieve not only technological advancements but also good economic feasibility and broad prospects for engineering applications.

[0021] (4) The present invention effectively solves the common problem of low efficiency of anaerobic digestion of sewage sludge in cold regions of my country and under seasonal low temperature conditions. It not only helps to achieve more thorough reduction, harmless treatment and resource utilization of sewage sludge, and reduce its potential pressure on the environment, but also can efficiently recover clean bioenergy - methane from it. It has important practical significance and positive social and economic value for promoting the development and utilization of renewable energy in cold regions of my country.

[0022] The present invention can obtain a method for improving methane production by low-temperature anaerobic digestion of sewage sludge. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1The methane yield changes under the conditions of adding different concentrations of corn straw biochar in the present invention are shown in Figure 2.

[0024] Figure 2 The methane yield changes under the conditions of adding different concentrations of mixed straw biochar in the present invention are shown in Figure 2. DETAILED DESCRIPTION

[0025] Specific embodiment 1: This embodiment provides a method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge, characterized in that the method is carried out in the following steps:

[0026] Step S1, preparing a mixed matrix:

[0027] Adding the inoculum and culture medium into the reaction device, stirring and mixing them thoroughly to obtain a mixed matrix;

[0028] The inoculum is anaerobic digested sludge, and the culture medium is sewage sludge that has been heat-treated at 90-100°C for 0.5-1 h;

[0029] This heat treatment method effectively releases amino acids, polysaccharides, and other small molecule metabolites into the liquid phase, significantly increasing the biodegradability of organic matter in the sludge and subsequently improving the efficiency of anaerobic digestion. Furthermore, heat treatment also has a certain effect in inactivating pathogens, enhancing the sanitation and safety of the treatment system.

[0030] Step S2, low-temperature anaerobic digestion to produce methane:

[0031] The biochar is added to the mixed matrix obtained in step S1 to perform low-temperature anaerobic digestion, thereby improving the methane production by low-temperature anaerobic digestion of sewage sludge.

[0032] Beneficial effects of this embodiment:

[0033] (1) This embodiment successfully overcomes the inhibitory effect of low temperature on microbial metabolic activity by innovatively introducing biochar into the low-temperature anaerobic digestion system and combining it with an optimal substrate pretreatment method. As shown in Example 1, under the conditions of optimal biochar dosage, the methane yield of the system increased significantly by 254.8% compared to the blank control group without biochar addition, indicating a breakthrough improvement in the energy conversion efficiency of sewage sludge under low-temperature conditions.

[0034] (2) The biochar added in this embodiment not only serves as an excellent carrier for microbial attachment, but more importantly, it serves as an efficient exogenous electron mediator, which can significantly improve the activity of key metabolic enzymes in the anaerobic digestion system, optimize and regulate the community structure and abundance of core functional microorganisms such as acid-producing bacteria, methanogens, and electroactive bacteria, promote the synergistic effect between microorganisms and the efficiency of direct / indirect electron transfer, and build a more stable and efficient microbial metabolic network, thereby greatly enhancing the operational robustness and shock resistance of the entire anaerobic digestion system under adverse conditions such as low temperature.

[0035] (3) The technical solution proposed in this embodiment has clear steps and a simple and easy operation process. It is highly compatible with existing sewage sludge anaerobic digestion facilities and can be applied and promoted without large-scale or complex equipment modifications. At the same time, biochar, as the core strengthening material, is widely available and relatively inexpensive. This allows the method of this embodiment to achieve not only technological advancements but also good economic feasibility and broad prospects for engineering applications.

[0036] (4) This implementation method effectively solves the common problem of low efficiency of anaerobic digestion of sewage sludge in cold regions of my country and under seasonal low temperature conditions. It not only helps to achieve more thorough reduction, harmless treatment and resource utilization of sewage sludge, and reduce its potential pressure on the environment, but also can efficiently recover clean bioenergy - methane from it. It has important practical significance and positive social and economic value for promoting the development and utilization of renewable energy in cold regions of my country.

[0037] Specific embodiment 2: The difference between this embodiment and specific embodiment 1 is that the volume ratio of the inoculum to the culture medium in step S1 is (1.22-1.86):1.

[0038] The other steps are the same as those in the first embodiment.

[0039] Specific embodiment three: This embodiment differs from specific embodiment one or two in that the reaction device described in step S1 uses an anaerobic bottle with a volume of 100 mL.

[0040] The other steps are the same as those in the first or second embodiment.

[0041] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that: in step S1 , the temperature is 19-21° C. and the rotation speed is 145-155 rpm.

[0042] The other steps are the same as those in Specific Embodiments 1 to 3.

[0043] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that the ratio of the mass of the biochar to the volume of the mixed matrix in step S2 is (0.25-5) g:1 L.

[0044] The other steps are the same as those in Specific Embodiments 1 to 4.

[0045] Specific embodiment six: The difference between this embodiment and specific embodiments one to five is that the ratio of the mass of the biochar to the volume of the mixed matrix is ​​0.5g:1L, the biochar is corn straw biochar or mixed straw biochar, the mixed straw biochar is composed of corn straw, wheat straw, peanut straw and sawdust, and the mass ratio of corn straw, wheat straw, peanut straw and sawdust is 1:1:1:1.

[0046] All of the above raw materials (corn straw, wheat straw, peanut straw, and sawdust) were oven-dried at 80°C to constant weight, pulverized, and sieved through a 1 mm mesh. The treated raw materials were then pyrolyzed in a tube furnace under continuous nitrogen flow. The specific pyrolysis conditions are as follows:

[0047] Corn straw biochar: heated to 500°C, heating rate of 10°C / min, pyrolysis time of 2 hours;

[0048] Mixed straw biochar: heated to 900°C, heating rate 6.5°C / min, pyrolysis time 4 hours;

[0049] Nitrogen was continuously introduced during pyrolysis to maintain an anaerobic environment. After pyrolysis, the samples were cooled to room temperature. The resulting biochar samples (corn straw biochar or mixed straw biochar) were thoroughly ground with a mortar and pestle and sieved through a 200-mesh sieve to obtain powdered biochar with a particle size less than 75 μm for subsequent low-temperature anaerobic digestion experiments.

[0050] The other steps are the same as those in Specific Embodiments 1 to 5.

[0051] Specific embodiment seven: This embodiment differs from specific embodiments one to six in that the initial pH of the solution at the start of low-temperature anaerobic digestion in step S2 is 7-8.

[0052] The other steps are the same as those in Specific Embodiments 1 to 6.

[0053] Specific embodiment eight: This embodiment differs from specific embodiments one to seven in that the temperature of the low-temperature anaerobic digestion in step S2 is 19-21°C.

[0054] The other steps are the same as those in Specific Embodiments 1 to 7.

[0055] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that the rotation speed of the low-temperature anaerobic digestion in step S2 is 120-150 rpm / min.

[0056] The other steps are the same as those in Specific Embodiments 1 to 8.

[0057] Specific embodiment ten: This embodiment differs from specific embodiments one to nine in that the total time of low-temperature anaerobic digestion in step S2 is 30 to 40 days.

[0058] The other steps are the same as those in Specific Embodiments 1 to 9.

[0059] The following examples are used to verify the beneficial effects of the present invention:

[0060] Example 1: A method for improving methanogenesis in low-temperature anaerobic digestion of sewage sludge, comprising the following steps:

[0061] Step S1, preparing a mixed matrix:

[0062] 65 mL of inoculum and 35 mL of culture medium were added to a 100 mL anaerobic bottle and mixed thoroughly at 20 °C and 150 rpm to obtain a mixed matrix;

[0063] Anaerobic digested sludge was used as the inoculum, and sewage sludge heat-treated at 90°C for 1 hour was used as the culture medium. The volume ratio of anaerobic digested sludge to sewage sludge was 1.3:1 to prevent the accumulation of volatile fatty acids (VFAs). This prevents acidic conditions from inhibiting methanogens, which could cause a sudden drop in methane production, stagnation of the digestion process, and even system collapse.

[0064] Step S2, low-temperature anaerobic digestion to produce methane:

[0065] Solid biochar was added to the mixed matrix and digested at low temperature for 35 days at a temperature of 20°C and a rotation speed of 150 rpm / min. The initial pH of the solution at the beginning of low temperature anaerobic digestion was 7-8.

[0066] Blank group: No biochar was added, and other conditions were the same as in Example 1.

[0067] The gas produced in the reaction device was exhausted every 2-4 days, and the exhaust volume was recorded. A gas chromatograph (GC7890A) was used to collect the biogas phase change data and calculate the cumulative methane production.

[0068] Experimental part:

[0069] During the low-temperature anaerobic digestion process of this embodiment, the methane yield change was continuously measured. The methane yield corresponding to the conditions of adding different concentrations of corn straw biochar was as follows: Figure 1 As shown in the figure, the maximum methane yields corresponding to biochar concentrations of 0.25 g / L, 0.5 g / L, 1 g / L, 5 g / L, 10 g / L, 15 g / L and 20 g / L were 188.1%, 238.1%, 254.8%, 159.5%, 121.4%, 90.5% and 85.7% higher than those of the blank group, respectively. Figure 2 As shown, 0.5 g / L mixed straw biochar (corn straw, wheat straw, peanut straw, and wood chips mixed in a 1:1:1:1 ratio) had the highest methane yield, increasing by 57.1% compared to the blank control, followed by 1 g / L, with a 47.6% increase. The difference in methane production between the two biochars after addition is primarily attributed to the higher content of oxygen-containing functional groups (OFGs) in corn straw biochar. These functional groups enhance the redox activity of the microbial community (12-15% higher than mixed straw biochar) and the activity of the electron transport system (76.5% higher), significantly promoting methane production during anaerobic digestion.

[0070] The results showed that the optimal corn straw biochar concentration was 1 g / L, and the highest methane yield was 7.6 mL / g VS / d, which was 245.5% higher than that of the blank group (2.2 mL / g VS / d).

[0071] Depend on Figure 1 It can be seen that compared with the methane yield of the blank group, the addition of biochar to the mixed matrix in this example can effectively increase methane production, which shows that the addition of biochar can effectively improve the anaerobic digestion performance of sewage sludge at low temperature, and is beneficial to increasing the methane production during the low-temperature anaerobic digestion of sewage sludge.

[0072] Comparative Example 1:

[0073] In the current study, Jang et al. [1]Anaerobic digestion using cow dung biochar as a substrate revealed that the addition of 10 g / L of cow dung biochar at 20°C resulted in a maximum methane yield of 24.3 mL / g VS / d, a 20.3% increase compared to the blank control (20.2 mL / g VS / d). Compared to Comparative Example 1, despite the higher organic matter content of the cow dung substrate used in Comparative Example 1 compared to the sewage sludge in Example 1, the maximum methane yields in both the blank and experimental groups were higher than those in the corresponding groups in Example 1. However, the superior methane yield increase in Example 1 was attributed to the biochar used in Example 1, which possesses abundant surface oxygen-containing functional groups and excellent electron transfer capacity, overcoming the issues of poor system stability and low methane production at low temperatures. Furthermore, the addition of biochar in Example 1 not only enhanced the richness and diversity of functional microorganisms, strengthening the construction of a more robust microbial metabolic network, but also promoted the synergistic effects of acidogens, methanogens, and electroactive bacteria, resulting in higher electron transfer activity and thus promoting methane production. Therefore, in general, Example 1 has a stronger promoting effect on low-temperature anaerobic digestion.

[0074] [1] Jang HM, Choi YK, Kan E. Effects of dairy manure-derived biochar on psychrophilic, mesophilic and thermophilic anaerobic digestions of dairy manure [J]. Bioresource Technology, 2018, 250: 927-931.

Claims

1. A method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge, characterized in that The method proceeds as follows: Step S1, preparing a mixed matrix: Adding the inoculum and culture medium into the reaction device, stirring and mixing them thoroughly to obtain a mixed matrix; The inoculum is anaerobic digested sludge, and the culture medium is sewage sludge that has been heat-treated at 90-100°C for 0.5-1 h; Step S2, low-temperature anaerobic digestion to produce methane: The biochar is added to the mixed matrix obtained in step S1 to perform low-temperature anaerobic digestion, thereby improving the methane production by low-temperature anaerobic digestion of sewage sludge.

2. The method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that The volume ratio of the inoculum to the culture medium described in step S1 is (1.22~1.86):

1.

3. The method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that The reaction device described in step S1 uses an anaerobic bottle with a volume of 100 mL.

4. The method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that In step S1, the temperature is 19-21° C. and the rotation speed is 145-155 rpm.

5. The method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that The ratio of the mass of the biochar to the volume of the mixed matrix in step S2 is (0.25-5) g:1 L.

6. A method for improving methanogenesis in low-temperature anaerobic digestion of sewage sludge according to claim 1 or 5, characterized in that The ratio of the mass of the biochar to the volume of the mixed matrix is ​​0.5g:1L. The biochar is corn straw biochar or mixed straw biochar. The mixed straw biochar consists of corn straw, wheat straw, peanut straw and sawdust. The mass ratio of corn straw, wheat straw, peanut straw and sawdust is 1:1:1:

1.

7. The method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that The initial pH of the solution at the beginning of the low-temperature anaerobic digestion in step S2 is 7-8.

8. The method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that The temperature of low-temperature anaerobic digestion in step S2 is 19-21°C.

9. The method for improving methanogenesis by low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that The rotation speed of the low-temperature anaerobic digestion in step S2 is 120-150 rpm / min.

10. The method for improving methanogenesis in low-temperature anaerobic digestion of sewage sludge according to claim 1, characterized in that The total time for low-temperature anaerobic digestion in step S2 is 30-40 days.

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